I made a flood coolant tray for my Sherline mill using a loaf tin (I think it was a 2lb size).
A square of aluminium plate is bolted through the tray to the milling table with a little bit of silicone around the screws to seal them. The milling vise is then mounted to the aluminium plate. The bottom of the tin seems flat and uniform, so the top plate screws down pretty solidly.
I did a bit of overkill on the drain connection, making a plastic fitting and a couple of wedge-shaped rings so the fitting was horizontal, even though the side of the tin is sloped.
It works really well, and makes a world of difference when milling steel. It also helps contain the chips in the tray (I normally use an air blast for cleaning, which spits the chips everywhere).